1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly for optimization of dielectric circuit board materials for improved performance in RF filters.
2. Description of the Related Art
Microstrip and stripline radio frequency (RF) filters are commonly manufactured on specially designed substrate boards. One type of RF filter is a stepped impedance filter. A stepped impedance filter utilizes alternating high impedance and low impedance transmission line sections rather than primarily reactive components, such as inductors and capacitors, or resonant line stubs. Hence, stepped impedance filters are relatively easy to design and are typically smaller than other types of filters. Accordingly, stepped impedance filters are advantageous in circuits where a small filter is required.
Stepped impedance filters used in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places a stepped impedance filter on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the stepped impedance filter is covered with a dielectric substrate material. In a third configuration known as stripline, the stepped impedance filter is sandwiched within substrate between two electrically conductive (ground) planes.
Two critical factors affecting the performance of a substrate material are permittivity (sometimes called the relative permittivity or xcex5r) and the loss tangent (sometimes referred to as the dissipation factor). The relative permittivity determines the speed of the signal, and therefore the electrical length of transmission lines and other components implemented on the substrate. The loss tangent characterizes the amount of loss that occurs for signals traversing the substrate material. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to {square root over (Ll/Cl)} where Ll is the inductance per unit length and Cl is the capacitance per unit length. The values of Ll and Cl are generally determined by the physical geometry and spacing of the line structure as well as the permittivity of the dielectric material(s) used to separate the transmission line structures.
In conventional RF design, a substrate material is selected that has a relative permittivity value suitable for the design. Once the substrate material is selected, the line characteristic impedance value is exclusively adjusted by controlling the line geometry and physical structure.
The permittivity of the chosen substrate material for a transmission line, passive RF device, or radiating element influences the physical wavelength of RF energy at a given frequency for that line structure. One problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is optimized for all of the various passive components, radiating elements and transmission line circuits to be formed on the board. In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide respectively for practical implementation for a given substrate material. Since the physical size of the microstrip or stripline is inversely related to the relative permittivity of the dielectric material, the dimensions of a transmission line can be affected greatly by the choice of substrate board material.
An inherent problem with the foregoing approach is that, at least with respect to the substrate material, the only control variable for line impedance is the relative permittivity, xcex5r. This limitation highlights an important problem with conventional substrate materials, i.e. they fail to take advantage of the other factor that determines characteristic impedance, namely Ll, the inductance per unit length of the transmission line.
Conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the permittivity. Accordingly, conventional dielectric substrate arrangements for RF circuits have proven to be a limitation in designing circuits that are optimal in regards to both electrical and physical size characteristics.
The present invention relates to an RF filter. The RF filter includes a substrate having a plurality of regions. Each of the regions has respective substrate properties including a relative permeability and a relative permittivity. At least one filter section is coupled to one of the regions of the substrate which has substrate properties different as compared to at least one other region of the substrate. Other filter sections can be coupled to other substrate regions having different substrate properties as well. For example, the permeability and/or the permittivity of the substrate regions can be different. At least one of the permeability and the permittivity can be controlled by the addition of meta-materials to the substrate and/or by the creation of voids in the substrate.
The RF filter can be a stepped impedance filter. At least one filter section includes a transmission line section having an impedance influenced by the region of the substrate on which the filter section is disposed. The transmission line section construction can be selected from the group consisting of microstrip, buried microstrip, and stripline. Further, the RF filter can include a supplemental layer of the substrate disposed beneath the filter section.